Circuit breakers typically provide protection against the very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a magnetic trip unit, which trips the circuit breaker's operating mechanism to open the circuit breaker's main current-carrying contacts upon a short circuit condition.
Modern magnetic trip units include a magnet yoke (anvil) disposed about a current carrying strap, an armature (lever) pivotally disposed near the anvil, and a spring arranged to bias the armature away from the magnet yoke. Upon the occurrence of a short circuit condition, very high currents pass through the strap. The increased current causes an increase in the magnetic field about the magnet yoke. The magnetic field acts to rapidly draw the armature towards the magnet yoke, against the bias of the spring. As the armature moves towards the yoke, the end of the armature contacts a trip lever, which is mechanically linked to the circuit breaker operating mechanism. Movement of the trip lever trips the operating mechanism, causing the main current-carrying contacts to open and stop the flow of electrical current to a protected circuit.
Currently, circuit breakers having a magnetic trip unit described above allow for adjusting the air gap distance between the magnet yoke and the armature to obtain different trip set points. The trip set point range offered by adjusting the distance between the magnet yoke and the armature is limited because a large trip set point range requires a large air gap adjustment range. Because available space is often limited, a smaller than desired adjustment range results. Furthermore, overcurrent protection at a low current trip setting (e.g., three times the rated current of the circuit breaker) is inhibited because the magnetically induced force acting on the armature isn't significant enough to trip the latch system.
Those skilled in the art will appreciate that the electrical load of a motor is characterized by a starting (run-up) current and a running current. The starting current averages about six times the full load current of the motor, but the peak of the first half cycle, the so-called “inrush” current, can reach values of up to twenty times the full load current. The lower overcurrent range for motor protection is commonly 3× the full load current of the motor.
It is necessary for such magnetic trip units to be reliable at a low overcurrent setting without altering the magnetically induced force acting on the armature at high overcurrent settings. In addition, it is desired that magnetic trip units offer a broader spectrum of overcurrent ranges (e.g., for use in motor protection), so that the breaker can offer a broader range to trip at different levels of overcurrent. It is also desired that the magnetic trip units be compact.